Global Positioning Satellite (GPS) navigation has become the standard for most military and civilian navigation applications. There exist in both military and civil sectors hundreds of thousands of GPS or Global Navigation Satellite System (GNSS) receivers that are susceptible to signal jamming or other signal obstructions, e.g., buildings or urban canyons. Civilian or Standard Positioning Systems (SPS) rely on a non-encrypted, globally accessible signal. Military or Precise Positioning Systems (PPS) rely on encrypted GPS signals and require specially equipped receivers with cryptographic keys. For example, the military P-code is a very long (seven days long) PRN code, which is encrypted into a Y-code. More recently encrypted military M-code signals are broadcast from the GPS satellites. Military-grade receivers are capable of operating in a spoof-resistant mode in which a receiver tracks an encrypted ranging code whose pattern is unpredictable except to compliant and keyed user equipment.
Generally navigation receivers use a non-encrypted, repeating coarse acquisition (CA) code to estimate timing to narrow the search to acquire the military signal. While the coarse acquisition code repeats frequently and is generally readily acquired, jamming or other signal degradation or obstruction can make it difficult to establish a suitable position or timing fix with just the coarse acquisition code. Thus, under jamming conditions, alternative methods of determining a particularly precise position/timing fix are needed to acquire or lock on the non-repeating military Y-code signal. Military users may prefer the PPS due to the spoof resistance and authorized access features provided via encryption.
Brute force searches or estimations based on combinations of ground reference station data and/or non GNSS data may be sufficient in some cases to determine a positional and timing fix to acquire the military GPS Y-code under jamming. One promising proposed system employs Low Earth Orbit (LEO) satellite signals to supplement or augment GPS acquisition or positioning. For example, U.S. Pat. No. 5,812,961 (which is incorporated herein by reference in its entirety) describes the use of angular velocity of a LEO satellite to calculate a location vector by combining carrier phases for both LEO and GPS satellites. Additional LEO aided GPS systems are described in U.S. Pat. Nos. 6,373,432 and 7,732,400 (which are incorporated herein by reference in their entirety) and employ ground reference stations to provide increased positioning accuracy. The '961, '432 and '400 patents, however, suggest the use of a LEO satellite signal only to assist in acquiring or maintaining a GPS lock and are generally limited to scenarios with substantially static receiver conditions.
Still another proposed LEO satellite augmented GPS system employs an Inertial Measurement Unit (IMU) to further aid in obtaining greater positional accuracy. For example, U.S. Pat. No. 7,489,926 (which is incorporated herein by reference in its entirety) teaches that a previous IMU inertial positional fix may be recalled such that the IMU acts like a positional “flywheel” feedforward for GPS tracking in both time and space to bridge gaps or signal degradation intervals between GPS tracking locks. The navigation receiver uses the previous GPS lock and corresponding IMU positional fix along with LEO satellite data to estimate positional fixes in signal coverage gaps between GPS signal locks, e.g., under jamming, in urban canyons or tunnels, or other areas with signal blockages. The '926 patent, however, is generally limited to use of an IMU with LEO satellite data in scenarios in which a GPS lock, positional fix and IMU calibrations have previously been obtained and does not address true “cold start” scenarios in which the receiver is without the benefit of such previously obtained information. As used herein, the term “cold start” refers to a scenario in which there is no a prior information about the time, position, and attitude (orientation).
Many of the previously proposed systems are computationally costly and impractical in many field scenarios due to the positional accuracy needed to acquire a military GPS Y-code signal. In particular, the difficulty of acquiring a military GPS signal lock can be highly dependent on corresponding vehicle dynamics, with high vehicle dynamics (i.e., greater than about 12 mph) presenting an intractable obstacle to convergence on a positional solution sufficiently accurate to acquire the nonrepeating military GPS Y-code using previous methods. Stated otherwise, prior navigation systems have heretofore been unable to achieve calculation of a changing search envelope of sufficient accuracy to obtain a lock on a military GPS Y-code under jamming conditions and at high vehicle dynamics. This has generally been due to the conflict between the degree of accuracy of the search envelope required, the rate at which that envelope is changing at high vehicle dynamics, and the computational cost or delay in calculating the appropriate search envelope with prior systems.
Accordingly, improvements are sought in cold start acquisition and navigation under GPS jamming and high-vehicle dynamics.